This invention relates in general to generating holes in composites, and in particular, to generating holes in composites with a rotary cutting tool, the composites comprising at least two materials having different moduli of elasticity.
In the past, it has been difficult to quickly and accurately drill holes in composite material, such as for example, aramid fiber/epoxy or other resin bonded laminates. Experience has shown that the aramid fibers, which have a smaller diameter and a higher tensile strength, elasticity, and shear resistance than the more conventional glass fibers used in similar composite compositions, tend to deflect and bend before being cut by the shearing action of the cutting edge of the drill. This action forces the fibers into the softer resilient resin matrix material, increasing radial compressive forces on the drill and the surrounding composite material. These compressive forces generate frictional heat which further softens the resin matrix, allowing the rough aramid fiber to further escape the shearing action of conventional drills. The heat generated also limits the drill speed and feed pressure so that the material surrounding the drilled hole is not damaged by excessive heating. Also, as a result of this characteristic of aramid fiber reinforced resin laminates, holes drilled by conventional means are often undersized holes with frayed or fuzzy edges, and mushrooming of the aramid fibers at the tool exit.
It is known to drill holes in aramid fibers/epoxy laminates using a special self-centering serrated drill. This drill includes clockwise spiraled serrations which extend backwardly and inwardly from the pointed drill end to a reduced diameter portion of the drill, and counterclockwise spiral serrations which extend backwardly and outwardly from this reduced diameter portion of the drill. In order to prevent an excessive heating of the composite when this serrated drill is used to generate holes in aramid fibers/epoxy composites, a low drilling speed must be used. Also, to obtain clean exit and entrance surfaces, this serrated drill must fully penetrate the laminate, that is, the entire serrated area must pass totally through the composite. After this initial penetration, the drill must be moved rapidly back and forth in its axial direction, again using the entire threaded, or "hour-glass" portion of the drill to clean the remaining uncut fibers from the laminate.